#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(ri,n) for(int ri = (int)(n-1); ri >= 0; ri--)
#define rep2(i,x,n) for(int i = (int)(x); i < (int)(n); i++)
#define repit(itr,x) for(auto itr = x.begin(); itr != x.end(); itr++)
#define rrepit(ritr,x) for(auto ritr = x.rbegin(); ritr != x.rend(); ritr++)
#define ALL(x) x.begin(), x.end()
using ll = long long;
using namespace std;

int main(){
  int n;
  cin >> n;
  map<int, int> mp;
  int mode = -1;
  int ans = 0;
  rep(i, n){
    int in;
    cin >> in;
    if(in != mode){
      mp[mode]++;
      if(mp[mode] == 2){
        ans++;
      }else if(mp[mode] > 2){
        cout << -1 << endl;
        return 0;
      }
      mode = in;
    }
  }
  mp[mode]++;
  if(mp[mode] == 2){
    ans++;
  }else if(mp[mode] > 2){
    cout << -1 << endl;
    return 0;
  }

  cout << ans << endl;
  return 0;
}